mckechnie



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Feb. 2, 1960 l. c. MoKEcl-INIE Re. 24,773

ELECTRO-HYDRAULIC POWER FEED original Filed Nov. 4, 1955 2 sheets-shed z 5"?774 affari, 4

INVENTOR. j??? M/ecrzfe Re. 24,773 Reissued Feb. 2, 1960 ELECTRO-HYDRAULIC POWER FEED Ian C. McKechnie, Pontiac, Mich., assignor to Elox Corporation of Michigan, Clawson, Mich.

Original No. 2,882,437, dated April 14, 1959, Serial No. 545,047, November 4, 1955. Application for reissue September 2, 1959, Serial No. 838,020

3 Claims. (Cl. 314-61) Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue speeltication; matter printed in italics indicates the additions made by reissue.

This invention relates to electro-hydraulic and electromechanical servo-mechanisms and to electro-hydraulic and electro-mechanical power feed mechanisms for machine tools in particular.

. More specifically, it relates to electrode feed mechanism for arc-machining apparatus. The arc-machining process (also called spark-machining and electricaldischarge machining) differs from other electrical metal working processes such as arc-piercing, welding, cutting and etching in that the machining action is based on the creation of a series of intermittent electrical discharges across a gap in the presence of a coolant.

In the use of such processes, the workpiece generally is maintained at positive potential and the electrode at negative potential for rapid machining and minimum tool erosion. For best accuracy and surface finish, vit is important that the duration of the discharge (as it approaches the arc stage) be held to a minimum, but it is also important that the off-time not be excessive thus `to avoid slowing the rate of metal removal.

In addition, it is now recognized that for best overall results so far as speed, accuracy and surface finish are concerned, it is vitally important that the discharge gap be maintained at all times of discharge in exact predetermined space relationship. Any discharges that occur at times when this precise gap distance is not maintained have an adverse effect on the quality of the work per. formed.

It is known also that, as the workpiece material is removed in the form of expulsed particles, these particles may form stringers (chains of adhered particles) ,which cause temporary shorts between the tool and workpiece and prevent discharging. Temporary short-circuits may also result from core bendingL--a deformation of the portion of the workpiece which projects into the electrode coolant passageresulting from strain relief.

It is therefore extremely desirable that the electrode be instantaneously twithdrawn from the workpiece whenever a short occurs and be advanced immediately after the short has been cleared toresume the machining process. Various ways of accomplishing this have been proposed and some are lin use. See, for example, Mc- Kechnie Patent No. 2,588,744 and my co-pending application Serial No. 260,002, now abandoned.

It is accordingly the primary object of my invention to provide an improved method and means for controlling the electrode feed in an arc-machining apparatus, although it is to-,be understood that my improvement is applicable to other types of machining apparatus.

. Other objects and advantages will become apparent aslthe description progresses. Referring now to the following specification, taken'together with the accompanying drawings, it will be seen that preferred embodiments In the drawings:

Fig. 1 is a schematic diagram of the electrical and hydraulic circuit of one form of my improved servo mechanism;

Fig. 2 is a somewhat more detailed schematic showing of the hydraulic components;

Fig. 3 is a schematic showing of a modified form of servo of the electro-mechanical type; and

Fig. 4 is a wiring diagram of the Fig. 3 apparatus.

Referring to Figs. l and 2, it may be seen that I have shown a workpiece 10 which is being or is about to be machined by an electrode 12. Coolant is preferably supplied to the electrical discharge zone through a tube 14 (Fig. 2), and the properrelationship between the electrode and the workpiece is maintained by means of the servo-mechanism about to be described.

The electrode 12 is supported in a holder 16 suitably mounted by means not shown such that it may be moved linearly to feed or withdraw the electrode with respect to the work. The means for reciprocating the holder 16 comprises a rack 18 (Fig. 1) carried by the holder which is engaged by a gear 20. i

The gear 20 is mounted on a common shaft 22 with a gear 24. The latter is meshed with a gear 26 which is, in turn, mounted on a common shaft 28 with a gear 30. The gear 30 engages a rack 32 carried by a piston rod 34. The piston rod 34 has pistons 36 and 38 on its respective opposite ends, which pistons reciprocate in the cylinder 40 in response to differential iiuid pressure in the conduits 42, 44.

The uid pressure for the system is generated by a double acting pump 46 of any suitable design driven by a motor (not shown). The pump is connected on its pressure side to opposite ends of the cylinder 40 by the conduits 42 and 44 respectively. The conduit 42 is also connected by a branch 48 with a balancing valve 52. The conduit 44 is similarly connected by means of a branch 50.

Return or suction conduits S4, 55, connect the valve 52 with the suction side of the pump, and a sump or pressure dome (not shown) may be provided if needed to prevent iiuctuations in the iiuid lines.

As shown in Fig. 2, the conduits 48 and 50 terminate inside the valvehousing in conically ared end portions 56, 58. A balance lever 60 supported at its central portion by a knife edge or equivalent device 62 is provided at its opposite ends with conical portions 64, 66, which are received in the flared ends 56, 58. The lever 60 is intended to balance on its central point such that the portions 64-66 will normally offer equal resistance to iiow of fluid from the conduits 48, 50.

Tipping of the lever 60 will tend to restrict or cut oif the flow from one conduit and reduce restriction of the other iwith action similar to the movement of a needle valve. Means for tipping the lever -60 comprises a pair of magnets 68, 70. These magnets each have a conventional core and iield coil and when energized will attract the lever `60 which in this case is the armature.

The means for controlling energization of the armatures now will be described. i

The main power supply for the apparatus is indicated by numeral 72. This is preferably a pulsed power supply and may be similar to that described in Matulaitis copeuding application Serial No. 459,703, which power supply furnishes pulsed D.C. power and has adjustments to vary the energy delivered, duration of pulse and offtime. The power supply 72 is connected on its negative side by wire 74 to the electrode 12 and thence by wire 76 to the magnet 70, through a voltage sensing impedance matching device 78. The positive side of the power supply 72 is connected by a wire 80 to a current sensing a conductive workpiece for eroding said workpiece, servomeans for continuously and automatically positioning said electrode in preselected space relationship with the workpiece comprising, a movable electrode holder, a hydraulic piston operatively associated with said holder such that movement of the piston eiects corresponding movement of the electrode, a cylinder enclosing said piston and a pump for supplying pressure iluid to both ends of said cylinder for maintaining pressure on both sides of said piston, a balancing valve having a pair of discharge orices connected respectively to each end of said cylinder, [a] balance means [lever] operatively associated with said orifices for alternately restricting ilow therefrom, magnetic means for controlling the position of said balance means [lever], and circuit means connecting said magnetic means with said gap such that said balance means [lever] is operably responsive to changes in gap current.

3. In an electrical-discharge-machining apparatus having an electrode and means for causing intermittent electrical discharge across a gap between said electrode and a conductive workpiece for eroding said workpiece, servomeans for continuously and automatically positioning said electrode in preselected space relationship with the workpiece comprising, a movable electrode holder, a hydraulic piston operatively associated with said holder such that movement of the piston effects corresponding movement of the electrode, a cylinder enclosing said piston and a pump for supplying pressure liuid to both ends of said cylinder for maintaining pressure on both sides of said piston, a balancing valve having a pair of discharge orifices connected respectively to each end of said cylinder, [a] balance means [lever] operatively associated with said orifices for alternately restricting ow there from, magnetic means for controlling the position of said balance means [lever], and circuit means connecting said magnetic means with said gap such that said balance means [lever] is operably responsive to changes in gap voltage.

References Cited in the iile of this patent or the original patent UNITED STATES PATENTS Manchester Sept. 11, 1956 

